In downhole logging operations based on irradiation of the formation with neutrons, log analysis is enhanced by data which deals with the thermal neutron capture cross section measurements of fluids making up the borehole fluid and the surrounding formation. The neutron capture cross section of an element involved in the borehole fluid or surrounding formation is in part dependent on the probabilistic nature and energy of the particles in question as well as the nature of the nucleus of the capture material. For instance, thermal neutron measurements show strong dependence on fluid salinity. The macroscopic thermal neutron capture cross section .SIGMA. is the effective cross sectional area of a unit volume of material necessary for thermal neutron capture and it involves the volume fraction weighted sum of the probabilities of the various elements which make up the substance within that volume. As a practical matter, .SIGMA. for the borehole fluid and also for the formation materials can be determined from elemental or neutron analysis of water samples produced from the borehole and the formation.
If formation fluids flow from more than one formation which differ in fluid .SIGMA., the average fluid at the surface may not yield a meaningful value. But the present system can be used to take data along a well and may well locate a change in .SIGMA. with changes in formation fluids and thereby locate a specific type of production fluid. Assumptions involved in this measurement are primarily that the production fluids are homogeneous relative to the tested samples, and hence the samples are reliable. Heretofore, measuring tools which have been responsive to borehole fluid have included measuring chambers for holding borehole fluid. One such arrangement is shown in U.S. Pat. No. 4,500,781 where borehole fluid is directed from the borehole into a measurement chamber involving a neutron source at one end and a thermal neutron detector at the opposite end of the chamber. To the extent that a measuring chamber is involved, such a structure uses a fluid flow diverter which diverts continuous fluid flow toward the interior of the tool from the adjacent annular space to assure that the fluid making up the measured sample is consistent with the fluid in the borehole. This typically involves some kind of fluid diverter to assure sample fluid replenishment in the test volume. However, this typically limits a fluid flow velocity and hence limits the velocity of the tool. Always, the measuring chamber will contain less than the total volume in the cased well and that might otherwise be significant to the borehole fluid measurements. To the extent that some of the neutrons are scattered into and return from the surrounding pipe wall, cement and formation, the capture of materials in those adjacent formations attributes some error to the sample volume in the measuring tool. In other words, measurements relating to borehole fluid are in error dependent on the contribution of the capture cross section measurement of surrounding materials making up the pipe, cement, formation, etc. To some extent, .SIGMA. for the formation results in changing the borehole measurements
One purpose of the present disclosure is to set forth a measuring device which has reduced sensitivity to the surrounding environmental structure. The present apparatus is therefore a logging tool which provides a macroscopic thermal neutron capture cross section measurement with markedly reduced error. Moreover, the present system can be used in a single detector mode in instances where environmental impact on the macroscopic thermal neutron capture cross section measurement is small. Where environmental impact is somewhat larger, first and second detectors can be used so that comparisons can be obtained to thereby reduce the impact of the environment outside the casing on the measurements. Further, this is accomplished in a tool which is preferably used in a centralized location in the borehole namely, a tool which is devoid of a sample storage chamber has no flow diverter.
While the foregoing is directed in general to some of the background and describes the present invention, a greater understanding will be obtained upon a review of the below written specification in conjunction with the drawings which follow.